Due to the increasingly strict gaseous pollutant emission regulations in many countries, and users' demands on performance and fuel consumption efficiency of motorcycles, the use of fuel injection technology in motorcycle engines is an important and emerging trend. However, the adoption of fuel injection technology inevitably increases the manufacturing cost of the engine and in turn the price of motorcycles. Furthermore, it is impossible to rapidly replace all old motorcycles in existing use with newer and cleaner ones in some countries. As such, there is a need to retrofit or modify traditional motorcycle engines that are still using the carburetor technology with the cleaner fuel injection technology. To be economically and logistically feasible, however, the retrofitting or modification must be simple and low cost.
There exist methods to replace a carburetor with a fuel injector with relative ease. However, the higher fuel injection and ignition timing requirements of a fuel injector-based engine must be met. One type of fuel injection-based engine uses crankshaft and camshaft dual position sensors to generate pickup signals, which can indicate crank angle, crankshaft rotational speed, piston and valve positions, and the stroke of combustion cycle, for an engine control unit (ECU) to precisely time the fuel injection and ignition systems. Another type of fuel injection engine uses a crankshaft having multiple rotational position marker teeth or notches (multi-tooth crankshaft) for a crankshaft position sensor to generate multiple pickup signals per rotation for the ECU. On the other hand, because of the relatively simpler mechanical design, a traditional carburetor-based engine has a single notch crankshaft for providing the timed triggers for ignition sparks. As such, its crankshaft position sensor can generate only one pickup signal per rotation, and by itself insufficient to accurately time the fuel injection and ignition systems. One solution is to install additional sensors but this increases the cost and further complicates the retrofitting. Substantially modifying the crankshaft or other parts of an old used engine is also not economically sound. Therefore, there is a need for a simple and low cost approach to retrofit carburetor-based motorcycle engines with fuel injectors. U.S. Pat. No. 6,955,081 and U.S. Pat. No. 6,804,997 disclose two engine control apparatuses for determining crankshaft position, engine phase, engine loading, and intake air mass of an internal combustion engine through monitoring intake air pressure fluctuations. The techniques employed in these engine control apparatuses, however, rely primarily on monitoring intake air pressure fluctuation during normal combustion cycles and does not have any facility to account for sudden changes such as abrupt engine acceleration and deceleration and engine start. During these sudden changes, the margin of error in the measurement of intake air pressure can be so significant that renders it unreliable for determining crankshaft position, engine phase, engine loading, and in turn engine ignition timing. Therefore, without a failure redundancy mechanism that does not require substantial modification or addition such as additional sensors or multi-tooth crankshaft angular position sensor, to a basic internal combustion engine, neither the '081 patent nor the '997 patent presents a simple and low cost approach to retrofit carburetor-based motorcycle engines with fuel injectors. U.S. Pat. No. 4,893,244 discloses a microprocessor based electronic spark control for an internal combustion engine. The spark control determines engine ignition spark time using a prediction of engine speed. This prediction model, however, is designed based on multi-tooth crankshaft angular position sensing that can generate numerous pickup signals for each combustion cycle. As mentioned earlier, while multi-tooth crankshafts are commonly found in multi-cylinder car engines, it is unusual for multi-tooth crankshafts to be used in motorcycle engines, especially those below 500cc. Therefore, the '244 patent does not present a solution for retrofitting carburetor-based motorcycle engines with fuel injectors either.